electrical machines for 2nd
TRANSCRIPT
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Electrical Machines
Fundamentals
Synchronous Machines
Induction Machines
Other Machines
Fundamentals
Design Aspects
Starting
Parameters
Operation
Introduction
Construction
Principles of Operation
Equivalent Circuit
Power & Torque
Torque-Speed Curve
Peak Torque & Power
Examples
Principles of Operation
The basic idea behind the operation of an induction machine is quite simple. Detailed mathematical
understanding of the interaction of magnetic fields and resultant torque is more complex. In many
cases, understanding the qualitative ideas and then applying a circuit model is sufficient. The qualitative
description is provided here, together with a more mathematical description for those who prefer that
type of approach. The mathematical description also includes some important definitions that are
required in order to develop a circuit mode.
Qualitative Description
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The three-phase stator winding is connected to a three-phase supply
Currents flow in the stator winding, producing a rotating mmf and flux density
The stator flux density rotates at synchronous speed:
The magnetic field passes conductors on the rotor and induces a voltage in those conductors
Since the conductors are short circuited, current flows in the rotor conductors
The rotor currents produce a second rotor magnetic field, which acts to oppose the stator magnetic field
and also rotates at synchronous speed
With two magnetic fields rotating at constant speed, a torque is induced:
The rotor flux density will lag the stator flux density (flux density lags current by 90 electrically),
therefore the torque will be in the same direction as the rotation of the magnetic fields
The torque accelerates the rotor until synchronous speed is reached, at which time there is no relative
motion between the conductors and the stator flux density. Since the relative velocity is zero, the
induced voltage, rotor currents and flux density fall to zero and torque is also zero
Mathematical Principles
Definitions
supply frequency: fe in Hz or e in rad s-1
synchronous speed (radians per second): s
synchronous speed (revolutions per minute): ns
rotor mechanical speed (radians per second): m
rotor mechanical speed (revolutions per minute): nm
slip: difference between synchronous and mechanical speeds divided by synchronous speed:
slip speed:
in rpm: sns = ns-nm
in rad s-1: ss = s-m
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slip frequency: fsl = sfe in Hz or sl = se in rad s-1
Induced Rotor Voltage
As illustrated in the fundamental theory on rotating fields, passing balanced three-phase currents
through a balanced three-phase winding can produce a rotating mmf wave. Speed of rotation is set by
supply frequency and the number of poles in the machine. In an induction machine the air gap of the
machine is designed to be constant, therefore the rotating mmf will produce a rotating flux density. The
stator flux density can be defined in terms of either mechanical or electrical quantities:
In the above equation m, e are arbitrary phase angles in mechanical and electrical angles
respectively. We will set these to zero. is the location at which the flux density waveform is observed.
(At a given location, the flux density varies sinusoidally with time. At a given time, the flux density varies
sinusoidally with location.) Now, to understand how an induction machine works, we need to consider
the flux density seen by a conductor on the rotor.
In the image shown above, there is a rotor conductor at position m = . If the rotor is stationary thenthe rotor will observe the the stator flux density as
However, if the rotor is rotating at mechanical speed m the location of the conductor becomes
and the flux density seen by the conductor is given by
Now, the voltage induced in a conductor of length l moving perpendicular to a magnetic field is given by
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and the relative velocity of the conductor through the magnetic field is given by
Therefore the voltage induced in the conductor is given by
Two important results can be seen in the above equation:
The induced voltage is proportional to slip
The frequency of the induced voltage is proportional to slip
Rotor Currents and Field
Current Magnitude and Phase
Without knowing the full details of the rotor circuit, we can makes some assumptions about the circuit
to enable us to understand the behaviour of the induction machine. We will assume that the rotor
conductor is part of a circuit with constant resistance RR and inductance LR. (We will see later that
resistance can actually vary with slip, but will assume that it is constant for now). Now, if slip is low (s
0) then the reactance associated with the inductance will be negligible:
In this case, though induced voltage is small, the induced currents may be significant since the
conductors are short circuited, so RR is low. Also the currents will be approximately in phase with the
induced voltage. If slip is high (s 1) then the rotor reactance will be significant. Due to the increase in
induced voltage rotor currents will be high, but will lag the induced voltage significantly due to the
inductance of the rotor circuit.
Rotor Flux Density
We know that the flux density produced by a set of ac currents rotates at a speed given by
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In the case of the rotor currents, the above equation gives the speed of rotation relative to the
conductors. The actual speed of rotation of the flux density will be given by
i.e. the rotor magnetic field rotates at synchronous speed. We can get an understanding of the relative
position of the rotor and stator fields by drawing phasor diagrams. The phasor diagram of the stator flux
density phasor can be drawn from either a stator reference frame, where it rotates at electrical speed
e, or from a rotor reference frame, where it rotates at electrical speed se
First consider the case where slip is low. Induced current lags induced voltage slightly, the rotor flux
density is almost 90 electrically behind the stator flux density.
From
at low slips, the angle between flux density phasors is close to 90 and the torque will be approximately
proportional to induced voltage and therefore proportional to slip.
Now consider the case where slip is close to 1.0, mechanical speed is close to zero. In this case, rotor
current lags induced voltage and the angle between rotor and stator flux densities is much greater.
From the torque equation, even though the magnitude of the induced currents is higher and the rotor
flux density phasor has a larger magnitude, torque will not necessarily be higher than it is a low slips.
Summary
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Induction machines produce torque at all speeds except synchronous speed
Induction machines can operate with only one electrical source, they do not require a source to be
connected to the rotor.
Rotor currents and torque are nonlinear functions of slip, a measure of the relative speed between the
stator magnetic field and the rotor mechanical speeds
Andy Knight Report an Error 08-Jan-2012 8:49 PM
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Basic principle of induction machine(motor and generator)- simple description
An induction machine is the most simple electrical machine from constructional point of view, in most
of the cases. It can be classified into motor and generator. In this post, I want to discuss the
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characteristic common to both of these. Detailed description of each will be available soon in other
posts.
Induction machines work on induction principle, in other words it depends on Faraday's law of
induction (i.e. when a conductor moves in a magnetic field, it gets some voltage(induced voltage). this
voltage can set up current if construction permits and can set up its own magnetic field.). In this case
it should be noted that moving in a magnetic field actually makes the magnetic flux changing to the
moving conductor(actually seems to be changing, from the view point of one who is moving), and this
changing magnetic field causes voltage and current to be induced on the moving body.
But if the magnetic filed is itself changing in nature, then it can
induce voltage on a stationary conductor. This is the case for induction motor and generator. Motor
remains stationary(rotor of the motor), a changing voltage(i.e. magnetic flux) is supplied to the stator
and hence the rotor get some induced voltage because it remains stationary in changing magnetic
field. This rotor voltage creates rotor current and rotor magnetic field(rotor flux), this rotor flux try to
catch stator flux and thus rotor starts to rotate.
The case is not this simple in practice, but it is indeed the principle of rotation in induction machine.
When a voltage is supplied to the stationary coils(stators) , it creates a stator magnetic field. If the
voltage is AC, then magnetic flux created by it is changing in nature. So stator produces a magnetic
field variation and rotor get some induced voltage. For squirrel cage induction machine, end rings
make the path for current flow and for wound rotor machine, external resistance or simply wire is
used to provide current path. This current path allows rotor flux to be formed.
Stator flux can be thought of a man who is holding out his hand for someone to catch(and of course
running because of changing(AC) stator voltage), and rotor flux can be thought of a lonely man in an
island. When this lonely man see another man(stator flux) passing him with a stretched hand, he
instantly reacts to catch the hand and starts running to the first man. But due to some causes(will be
discussed in other posts), this lonely man can never reach the hand, but remains only a few inches
behind the stretched out hand. So he can never lost the hope of catching the hand and continues to
run as long as the hand remains stretched for him(i.e. as long as the stator is supplied with variable
voltage) . So the rotor of the motor tries to catch stator flux and hence rotates. (This is the basic idea
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of slip in induction machines, you can read deails about the slip in the following post: http://get-a-
solution.blogspot.com/2012/01/slip-of-induction-machine-hidden-power.html)
From the above discussion it should be noted that the stator flux (voltage also) must be AC for the
rotor to rotate, so induction machine can run on AC only. And when acting as generator, it will
generate AC directly.
In the case generator, rotor is rotated by external means(may be by turbine of some kind). If the rotor
has some residual magnetism(i.e. some magnetic properties which stores magnet type properties
inside the material, in a simple way to think about residual magnetism, but not describing it fully),
then the rotor is actually providing a variable flux to the stationary coils in the housing(stator coils). So
this stator coil will get some induced voltage on it by induction principle and we get some voltage to
supply our load or to store it in a battery. Induction generators are used in small shops andhouseholds to provide extra power support and are less costly due to easy construction. In recent
days, it is widely used by the people in those country where power authorities are bound to shed
some load periodically due to supply shortage. Most of the time, rotor is rotated by a small diesel
engine and the induction generator is coupled to it.
Fig: A fractional horse power induction generator
"It is the conceptualizations which are important." -A. S. Eddington, Fundamental Theory, 1944
The phenomena of electrical induction which are fundamental to electrical science have long since
passed into everyday experience. Recently the nature of this fundamental principle has been re-
examined in the light of experiments with electrical machines, which, in their operation violate the
conservation laws of charge and energy.
__________
In my early schooling (M.I.T. class of 1958) I was struck by the attention paid to magnetism, magnetic
circuits, electrical machinery and magnetic properties of materials. No attention was given to
magnetism as a source of understanding of the machines and apparatus which employed it. This
attitude was forced on a student because the consensus was: all that needed to be known about
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magnetism was known because electrical machines obeyed the conservation laws. I.e. one way of
generating electricity was as good as another since all machine efficiencies could be "improved" or
designed up to the point of a maximum efficiency of 100%.
To point out that electrical efficiency measurements are based on the "mechanical equivalent of
heat", 746 watts/horsepower, measured with a calorimeter and paddles by James Watt (inventor of
the steam engine) in the late 18th century; a number suspect both in its relevance and accuracy, and
sensitivity to experimental vagaries, was heresy.
The concern of this paper is not with all the experiments which have demonstrated anomalous "over
unity" energy production, but with the operation of machines which clearly demonstrate violation of
energy and charge conservation laws through continuous production of electrical power in excess of
the electrical power used to drive and/or energize the machine.
The experimental performance of over-unity machines, the N-machine and Space Power Generators
are substantially covered in the literature and are not repeated here. References ( 1 - 7 )
The basic question is: do electrons flow in a conducting circuit impelled by magnetic forces, or, are the
electrons created in situ by the magnetic forces, collected by the conducting wire, and then impelled
to flow in the appropriate direction by the well known force interaction of electrons and magnetism?
Einstein treated electromagnetic induction as simply a relationship between two members, i.e. the
magnet and the wire. He would ask, "what is the point?"
The point is if we stop at Relativity as being the finest appreciation of the experimental situation we
would never inquire into the nature of magnetism.
If we consider the original flux cutting experiment of Faraday where a conducting wire is passed
through the field existing at the pole of a magnet we observe an electrical potential across the ends of
the wire as long as the wire is moving. Reversal of the direction of motion of the wire reverses the
polarity of the created electrical potential. If the potential created is applied to an electrical circuit
and current flows then a resistance to the applied motion ensues. (Lenz's Law). Here the question is: is
Lenz's Law a concomitant or a consequence of the production of electrical energy?
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It is not useful to discuss something as fundamental as magnetism at the level of inquiry we wish to
pursue without a model of the Universe. Tewari is one of the few researchers who has recognized
this. Reference ( 8 ).
Magnetism is similar to the gyroscope in that both effects are used in navigational apparatus which
depend on an element which retains its orientation either to an external reference, (Earth
magnetization), or to itself. What can we say of effects which have directional properties yet seem to
orient themselves only to each other or to themselves.
Obviously the magnet and the gyroscope are oriented to a force which does not have a geometric
extension into our 3 space. The clear implication is that the magnet and the gyroscope orient
themselves to the flow of time energy.
A model of the Universe can be represented by a vortex ring; in which space and time are
perpendicular to each other. Figures ( 1 & 2 ). The flow of time energy energizes our Universe. It is this
to which the magnet orientates. Figure ( 3 ). The magnet has the property of collimating and
concentrating the time energy flow.
Why is all this necessary? It is a consequence of a Universe created from nothing - the void.
In a Universe created from nothing, time extension is necessary so the Universe shall not re-collapse
in any instant called the NOW. Time extension exists over multiple instants, the sum of which equals
the lifetime of particles found in our 3 space. The quantum of time is the Instant.
Magnetism has nothing to do with iron and electrical solenoids per se. It is the property of these
instruments to orient to and concentrate the time energy flow.
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In our practical society it is customary to extract energy from the natural flows, i.e. water and wind. If
there was an invisible flow through a magnet or solenoid how could we extract the energy? Suppose
we were to construe a copper disc placed in front of a magnetic pole a la Faraday as a form of
propeller the pitch of whose blades could be changed by the application of an electrical potential
between the center and outer edge. The flow of time energy through the magnet would cause the
propeller to rotate like a fan blade in a current of air. The fan can be placed at either end of the
magnet, and, providing the pitch of the blades is maintained unchanged in magnitude or direction, it
will rotate in the same direction.
If mechanical power is extracted from the shaft or propeller disc then we would find it more difficult
to maintain the electrical polarization, i.e. more current would be required. If the rotating Faraday
disc apparatus is viewed as a transducer between the electrical power input required to polarize the
disc and the resultant mechanical shaft horsepower, then the conservation laws would say the
mechanical power out could never exceed the electrical power in. Of course these two quantities are
related through the mechanical equivalent of heat experiment with the paddles agitating water in a
calorimeter. Acting with the insight of Einstein we would say that experiments which produce
identical results, i.e. agitating water with paddles to produce warming versus mechanical input to a
machine which produces electricity which is converted to heat by a resistor immersed in water in a
calorimeter; are equivalent, thus the figure 746 watts = 1 mechanical horsepower derived from these
measurements is a true and reliable number for all the world to see.
We know a priori that no transducer or electrical machine can operate at greater than 100% efficiency
so then if we are slightly uncertain about the 746 watts/horsepower figure we can adjust the units toget the exact number right.
"Scientists" feel no guilt with introduction of certain "constants" because they are protected by the
conservation laws which are based on common sense which everyone knows is true.
If we return to the analogy of the fan and the magnet we might suppose that rotational drag effects
might exist adjacent to the rotor. The action of these drag effects would be to drag the magnet, i.e.cause it to rotate in the same direction as the disc. Clearly then a reduction in mechanical drag on the
rotor could be effectuated by attaching the magnet to the disc and allowing them to rotate together.
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Of course if we adhere to the Law of action and equal and opposite reaction then we would never try
such an experiment because we would expect the magnet to be acted on by a torque equal and
opposite to the shaft horsepower exiting the rotating disc.
It has been known for 100 years that the exciting magnet of a homopolar or Faraday disc motor or
generator exhibits no reaction torque to the mechanical forces generated by the polarized disc.
Reference ( 9 & 10 ).
Contemporary experiments have also shown the Faraday disc to be a superior motor or generator
when the fixed exciting magnet is attached to and rotates with it, thereby removing a constant drag
which is superimposed on the mechanical input, or output of the machine. * ( Ibid. Reference 4 ).
What has all this to do with electrical induction or flux cutting? Simply nothing.
A mistake was made in science 150 years ago through what Einstein identified as the Principle of
Equivalence and energy conservation laws based on physical conceptions of the 18th century. It was
the attempt of science to square the behavior of the one-piece Faraday disc machine with the
performance of two piece induction machines where magnetic flux lines were perpendicular to the
axis of rotation.
It simply turns out that the efficiency of a two-piece Faraday disc machine is close enough to that of
an equivalent two piece induction machine, about 1%, so that generic differences between the two
families of machines are concealed in the indeterminacy of the exact number for the mechanical
equivalency of heat. Reference ( 11 ). If the magnet is loosed and free to rotate with the disc, i.e. the
one-piece Faraday homopolar generator, then the true distinction in families of machines is revealed.
The one-piece Faraday machine is superior to the two piece induction machines both as generator or
motor.
Without trying to tangle the reader in the circularities and tautologies of modern scientific reasoning,
acceptance of a family of motors and generators without stators to receive reaction torques
contradicts Newton's third Law. We can avoid consideration of this problem by not using these sorts
of machines.
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Men are more persistent in their pursuit of inquiry. If a superior machine is found men will endeavor
to explain it. If a machine produces in excess of 746 watts per input horsepower what is our
interpretation of this "excess" energy production.
The Universe is alive and this is beyond our powers of conception. We can say, based on our
experience, a certain intellectual model can be constructed. This is like saying the world is round or
that the planets rotate in circles around the sun. Neither statement is exactly true, but they
rationalize information in our minds and lead to new knowledge.
We are familiar with the process of transmission and reception of electrical energy by means of
resonant structures known as antennas.
An antenna for the reception of Universal Energy would be a model of the Universe itself. The
suggested structure is the one-piece Faraday disc, homopolar generator. Figure ( 4 ).
The magnetic flux lines become the time lines of the space energy flow and the rotating disc is the 3
space Universe existing in the instant of the present.
As for the family of two piece induction machines, these are seen by this author to operate on the
principle of transformer induction, including d.c. machines which are nothing but transformers with
rotating secondaries and mechanical commutators for rectification.
A superior motor would produce more output power, torque x speed of rotation, per increment of
input electrical excitation. The output power would exceed 1 horsepower for 746 watts of electrical
input.
A superior generator would produce more than 746 watts electrical output per horsepower input.
A two-piece induction machine operating essentially as a rotating transformer would never be able to
exceed 100% electrical efficiency because electrical transformers in themselves are not known to be
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able to create energy. (There may be special circumstances where this is not true, but these peculiar
effects characterized by a negative are not normally encountered in conventional electrical machines)
The mirror image symmetry characteristic of the input and output ports of a transformer is carried
over to the equivalence of two-piece induction machines operated as motors or generators. This
motor-generator symmetry is not characteristic of the one-piece Faraday homopolar machine.
As a generator the one-piece homopolar machine evinces reduced drag in comparison with the two-
piece induction machine for the production of equal amounts of electrical power. This is because the
perceived mechanism of operation is to precipitate electrical charge from the time-energy flow by a
centrifugally engendered force field. Reference ( 12 ).
As a motor the one-piece homopolar machine produces the same amount of torque as an equivalent
two-piece induction machine for measurements made with a blocked rotor. Reference ( 13 ). The
reduction of magnetically induced drag by attachment of the magnet to the rotor is not evinced by
static measurements.
The torque attainable from a motor acts in relation to the Earth reference frame. For a two-piece
induction machine, the stator, the receptor of the reaction torque from the rotor, is physically
attached to the Earth reference frame. In contrast the one-piece homopolar machine has no fixed
Earth reference. With the rotor blocked there is a physical connection to the fixed Earth reference
frame and the relationship between motor torque vs. current input follows conventional
expectations.
With the magnet of the one-piece machine loosed to rotate with attached Faraday disc the
mechanical connection to a fixed Earth reference frame is broken. With this connection broken the
ability of this motor to do useful work is compromised by the necessity of transferring torque from a
rotating reference frame to a fixed one. As the one-piece machine rotates at increasingly higher
speeds the torque connection between the rotating frame and the fixed Earth frame becomes moretenuous until the torque output of the machine is balanced by mechanical losses. Further increases in
motor current result in increasingly disproportional torque to the point where no further current
increase can produce an increase in motor speed.
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References:
1) Kincheloe, 1986, "Homopolar `Free Energy' Generator Test"; paper presented at the 1986 meeting
of The Society for Scientific Exploration, San Francisco, California, June 21, 1986; revised February 1,
1987. Address: Dr. W. Robert Kincheloe, 401 Durand/ITV, Stanford, California 94305
2) DePalma, 1988, "Initial Testing Report of DePalma N-1 Electrical Generator"; Magnets in Your
Future, vol. 3, no. 8, August 1988, pp. 4-7, 27; P. O. Box 250, Ash Flat, Arkansas 72513, U. S. A.
3) Tewari, P. , "Generation of Electrical Power from Absolute Vacuum by High Speed Rotation of
Conducting Magnetic Cylinder"; Magnets in Your Future, vol. 1, no. 8, August 1986.
4) Tewari, P. , "Space Power Generation"; Magnets in Your Future, vol. 6, no. 8, August 1992.
5) Tewari, P. , "Generation of Cosmic Energy and Matter from Absolute Space (Vacuum)"; proceedings
of the International Symposium on New Energy, Denver, Colorado, U. S. A., April 16-18, 1993.
6) Inomata, S., and Yoshiyuki, M., "Small Neodymium Magnet Twin N-Machine"; proceedings of the
28th I.E.C.E.C., Atlanta, Georgia, U. S. A., August 8-13, 1993. Address: Dr. Shiuji Inomata, Japan
Electrotechnical Laboratory, MITI, 1-1-4 Umezono, Tsukuba-shi, Ibaraki, 305, Japan.
7) Laureti, E. , "Alcune Osservazioni sull'Induzione Unipolare"; Nova Astronautica, vol. 12, no. 54, pp.
27-33, 1992.
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8) Tewari, P. , Beyond Matter; Printwell Publications, Aligarh, India, 1984.
9) Kimball, A. L., Jr., "Torque on a Revolving Cylindrical Magnet"; Physical Review, vol. 28, December
1928, pp. 1302-1308.
10) Das Gupta, A. K. , 1963, "Unipolar Machines, Association of the Magnetic Field with the Field
Producing Magnet"; Am. J. Phys., vol. 31, pp. 428-430, 1963.
11) Private conversation reported by Adam Trombly with physicist developing superconducting
homopolar motors and generators for the U. S. Navy ship propulsion project, 1980. "I suppose only a
physicist would worry about this but the efficiency of the homopolar generator, (superconducting
two-piece), is 1% higher than calculated, 97% vs. 96%."
N.B- A. D. Trombly, Director of Research and Development, Zero Point Technologies Inc., P. O. Box
1031, Evergreen, Colorado, 80439, U. S. A.
12) DePalma, B. , "Magnetism as a Distortion of a Pre-Existent Primordial Energy Field and the
Possibility of Extraction of Electrical Energy Directly from Space"; proceedings of the 26th Intersociety
Energy Conversion Engineering Conference, I.E.C.E.C., sponsored by the I.E.E.E. ( U. S. A. ), August 4-9,1991. Boston, Massachusetts.
13) Crooks, Litvin, and Matthews, 1978, "One Piece Faraday Generator: A Paradoxical Experiment
from 1851"; Am. J. Phys., vol. 46, no. 7, July 1978, pp. 729-731.
From Wikipedia, the free encyclopedia
Jump to: navigation, search
The academic study of electric machines is the universal study of electric motors and electric
generators. By the classic definition, electric machine is synonymous with electric motor or electric
generator, all of which are electro-mechanical energy converters: converting electricity to mechanical
power (i.e., electric motor) or mechanical power to electricity (i.e., electric generator). The movement
involved in the mechanical power can be rotating or linear.
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Although transformers do not contain any moving parts they are also included in the family of electric
machines because they utilise electromagnetic phenomena.[1]
Electric machines (i.e., electric motors) consume approximately 60% of all electricity produced.
Electric machines (i.e., electric generators) produce virtually all electricity consumed. Electric
machines have become so ubiquitous that they are virtually overlooked as an integral component of
the entire electricity infrastructure. Developing ever more efficient electric machine technology and
influencing their use are crucial to any global conservation, green energy, or alternative energy
strategy.Contents
1 Classifications
1.1 Electromagnetic-rotor machines
1.1.1 Permanent magnet machines
1.1.2 Brushed machines
1.1.3 Induction machines
1.2 Reluctance machines
1.3 Electrostatic machines
1.4 Homopolar machines
2 References
[edit] Classifications
When classifying electric machines (motors and generators) it is reasonable to start with physical
principle for converting electric energy to mechanical energy. It is important to distinguish betweenthe machine and the controller regardless of the controller being a separate inverter or if it is built
into the motor in the form of a commutator. If the controller is included as a part of the machine all
machines can be powered by both AC and DC current, although some machines will need a more
advanced controller than others. Classification is complicated by the possibilities of combining
physical principles when constructing an electrical machine. It can, for example, be possible to run a
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brushed machine as a reluctance machine (without using the rotor coils) if the rotor iron has the
correct shape.
Generally all electric machines can be turned inside out, so rotor and stator exchange places. All
rotating electric machines have an equivalent linear electric machine where stator moves along a
straight line instead of rotating. The oppositelinear to rotary dualis not always the case. Motors
and generators can be designed with or without iron to improve the path of the magnetic field (teeth
to reduce the air gap is a common example) and with and without permanent magnets (PM), with
different pole number etc., but still belong to different classes of machines. Electric machines can be
synchronous meaning that the magnetic field set up by the stator coils rotates with the same speed as
the rotor; or asynchronous, meaning that there is a speed difference. PM machines and reluctance
machines are always synchronous. Brushed machines with rotor windings can be synchronous when
the rotor is supplied with DC or AC with same frequency as stator or asynchronous when stator and
rotor are supplied with AC with different frequencies. Induction machines are usually asynchronous,
but can be synchronous, if there are superconductors in the rotor windings.
[edit] Electromagnetic-rotor machines
Electromagnetic-rotor machines are machines having some kind of electric current in the rotor which
creates a magnetic field which interacts with the stator windings. The rotor current can be the internal
current in a permanent magnet (PM machine), a current supplied to the rotor through brushes
(Brushed machine) or a current set up in closed rotor windings by a varying magnetic field (Induction
machine).
[edit] Permanent magnet machines
PM machines have permanent magnets in the rotor which set up a magnetic field. The
magnetomotive force in a PM (caused by orbiting electrons with aligned spin) is generally much higher
than what is possible in a copper coil. The copper coil can, however, be filled with a ferromagnetic
material, which gives the coil much lower magnetic reluctance. Still the magnetic field created by
modern PMs (Neodymium magnets) is stronger, which means that PM machines have a better
torque/volume and torque/weight ratio than machines with rotor coils under continuous operation.This may change with introduction of superconductors in rotor.
Since the permanent magnets in a PM machine already introduce considerable magnetic reluctance,
then the reluctance in the air gap and coils are less important. This gives considerable freedom when
designing PM machines.
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It is usually possible to overload electric machines for a short time until the current in the coils heats
parts of the machine to a temperature which cause damage. PM machines can in less degree be
subjected to such overload because too high current in the coils can create a magnetic field strong
enough to demagnetise the magnets.
[edit] Brushed machines
Brushed machines are machines where the rotor coil is supplied with current through brushes in much
the same way as current is supplied to the car in an electric slot car track. More durable brushes can
be made of graphite or liquid metal. It is even possible to eliminate the brushes in a "brushed
machine" by using a part of rotor and stator as a transformer which transfer current without creating
torque. Brushes must not be confused with a commutator. The difference is that the brushes only
transfer electric current to a moving rotor while a commutator also provide switching of the currentdirection.
There is iron (usually laminated steel cores made of sheet metal) between the rotor coils and teeth of
iron between the stator coils in addition to black iron behind the stator coils. The gap between rotor
and stator is also made as small as possible. All this is done to minimize magnetic reluctance of the
magnetic circuit which the magnetic field created by the rotor coils travels through, something which
is important for optimizing these kind of machines.
Large brushed machines which are run with DC to the stator windings at synchronous speed are the
most common generator in power plants, because they also supply reactive power to the grid,
because they can be started by the turbine and because the machine in this system can generate
power at constant speed without a controller. This type of machine is often referred to in the
literature as a synchronous machine.
This machine can also be run by connecting the stator coils to the grid, and supplying the rotor coils
with AC from a inverter. The advantage is that it is possible to control rotating speed of the machinewith a fractionally rated inverter. When run this way the machine is known as a brushed double feed
"induction" machine. "Induction" is misleading because there is no useful current in the machine
which is set up by induction.
[edit] Induction machines
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Induction machines have short circuited rotor coils where a current is set up and maintained by
induction. This requires that the rotor rotates at other than synchronous speed, so that the rotor coils
are subjected to a varying magnetic field created by the stator coils. An induction machine is an
asynchronous machine.
Induction eliminates the need for brushes which is usually a weak part in an electric machine. It also
allows designs which make it very easy to manufacture the rotor. A metal cylinder will work as rotor,
but to improve efficiency a "squirrel cage" rotor or a rotor with closed windings is usually used. The
speed of asynchronous induction machines will decrease with increased load because a larger speed
difference between stator and rotor is necessary to set up sufficient rotor current and rotor magnetic
field. Asynchronous induction machines can be made so they start and run without any means of
control if connected to an AC grid, but the starting torque is low.
A special case would be an induction machine with superconductors in the rotor. The current in the
superconductors will be set up by induction, but the rotor will run at synchronous speed because
there will be no need for a speed difference between the magnetic field in stator and speed of rotor
to maintain the rotor current.
Another special case would be the brushless double fed induction machine, which has a double set of
coils in the stator. Since it has two moving magnetic fields in the stator, it gives no meaning to talk
about synchronous or asynchronous speed.
[edit] Reluctance machines
Reluctance machines have no windings in rotor, only a ferromagnetic material shaped so that
"electromagnets" in stator can "grab" the teeth in rotor and move it a little. The electromagnets are
then turned off, while another set of electromagnets is turned on to move stator further. Another
name is step motor, and it is suited for low speed and accurate position control. Reluctance machines
can be supplied with PMs in stator to improve performance. The electromagnet is then turned of
by sending a negative current in the coil. When the current is positive the magnet and the current
cooperate to create a stronger magnetic field which will improve the reluctance machines maximum
torque without increasing the currents maximum absolute value.
[edit] Electrostatic machines
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In electrostatic machines, torque is created by attraction or repulsion of electric charge in rotor and
stator.
[edit] Homopolar machines
Homopolar machines are true DC machines where current is supplied to a spinning wheel through
brushes. The wheel is inserted in a magnetic field, and torque is created as the current travels from
the edge to the centre of the wheel through the magnetic field.
[edit] References
^ Flanagan. Handbook of Transformer Design and Applications, Chap. 1 p1.
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